1. Field of the Invention
The present invention relates to compositions containing isocyanato-silanes as additives to increase the storage stability of liquid or pasty compositions containing polyurethane prepolymers with free isocyanate groups.
2. Description of the Prior Art
In German Patent No. 1,245,590 it has been disclosed to use highly reactive sulphonyl isocyanates, in particular tosyl isocyanate, as water binding agents in preparations containing polyurethane prepolymers with free isocyanate groups.
The use of such highly reactive monoisocyanates in lacquer systems has hitherto been the most advantageous means, for example, of preventing any undesirable side reactions with water possibly still present in the solvents or pigments. In practice, however, the use of tosyl isocyanate as an additive for lacquer compositions also entails disadvantages, as disclosed in German Offenlegungsschrift No. 2,539,728.
Thus commercial tosyl isocyanate occasionally contains traces of tosyl chloride which is also formed in the course of the preparation of tosyl isocyanate and which may give rise to unpleasant odors accompanying the use of tosyl isocyanate. Furthermore, the tosyl amide formed as a result of the reaction of tosyl isocyanate with water is liable to cause stippling in the lacquer due to the pronounced tendency of tosyl amide to undergo crystallization. In addition, tosyl amide often has a reaction inhibiting effect on polyurethane lacquer compositions.
Handling of tosyl isocyanate, which is a highly reactive compound, requires certain safety measures. Contact of tosyl isocyanate with pure water must be prevented as it may lead to an extremely vigorous reaction.
German Offenlegungsschrift No. 2,539,728 therefore recommends the use of certain alkoxycarbonyl isocyanates instead of tosyl isocyanate.
The disadvantage of alkoxycarbonyl isocyanates lies in the difficulty of their methods of preparation, for the most part requiring expensive starting materials. Thus in the process according to J. Org. Chem. 30 (1965), pages 4306 to 4307, preparation of the alkoxycarbonyl isocyanates requires the reaction of a previously prepared N-unsubstituted urethane with oxalyl chloride. The process according to German Auslegeschrift No. 1,793,088 requires the use of chlorocarbonyl isocyanate and the processes according to German Offenlegungsschrift No. 1,900,755 and German Auslegeschrift No. 1,768,179 require the use of chlorocarbonyl-isocyanide dichloride.
It was therefore an object of the present invention to provide an effective, easily available waterbinding additive for preparations containing polyurethane prepolymers with free isocyanate groups, which would not have the disadvantages mentioned above.
It has now surprisingly been found that isocyanato-silanes satisfy the requirements mentioned above.
The good drying effect of the isocyanato-silanes to be used according to the invention was not obviously to be expected since many representatives of this class of substances, e.g. trimethyl-isocyanatosilane, react only very slowly with water.
The reaction with water, for example in the case of dimethyldiisocyanatosilane, gives rise to polysiloxanes. These end products are constituents of mold release agents so that when prepolymers containing silyl isocyanate were subsequently used as lacquers, problems with adherence of the lacquers would be expected to arise.
It was surprisingly found, however, that isocyanato-silanes are excellent drying agents in isocyanate prepolymers and do not have the last mentioned disadvantage.